Abstract Background Immune checkpoint inhibitors (ICIs) have significantly improved survival outcomes in lung cancer but can also induce severe immune-related adverse events (irAEs) involving multiple organs. Neuromuscular and cardiac toxicities are rare but potentially fatal. We report a case of concurrent ICI-associated myasthenia gravis (MG) and myocarditis in a patient with small cell lung cancer (SCLC), highlighting diagnostic challenges and therapeutic strategies. Case Presentation A 44-year-old man with extensive-stage right lung SCLC received first-line etoposide plus carboplatin chemotherapy combined with Tislelizumab. Two weeks after treatment, he developed ptosis of the left upper eyelid without limb weakness. Physical examination revealed isolated left eyelid drooping. Laboratory analysis showed elevated myoglobin, creatine kinase, and troponin levels. Imaging demonstrated significant tumor reduction, suggesting a good oncologic response. During hospitalization, symptoms progressed to bilateral ptosis, dysphagia, and generalized weakness. Troponin levels further increased. Repetitive nerve stimulation revealed a decremental response in facial and accessory nerves. Anti-acetylcholine receptor and ryanodine receptor antibodies were positive. Based on multidisciplinary consultation, ICI-associated MG with myocarditis was diagnosed. Treatment included intravenous immunoglobulin (20 g/day × 5 days), high-dose and tapering methylprednisolone, and pyridostigmine bromide. Supportive care involved coenzyme Q10 and vitamin supplementation. Clinical symptoms improved significantly within two weeks, with complete recovery of ptosis and normalization of muscle strength. Troponin levels declined, and no respiratory involvement occurred. The patient was discharged after 19 days on a tapering steroid regimen and maintained outpatient follow-up. Discussion ICI-related neuromuscular disorders may present with overlapping features of myasthenia gravis and myositis, frequently accompanied by myocarditis. Early recognition and timely immunosuppressive therapy are crucial to prevent progression to respiratory or cardiac failure. The coexistence of MG and myocarditis represents a severe irAE requiring multidisciplinary management and drug discontinuation. Conclusion Clinicians should maintain vigilance for overlapping myasthenic and cardiac toxicities during ICI therapy. Prompt recognition, early initiation of corticosteroids and IVIG, and close monitoring can improve clinical outcomes. This case underscores the importance of comprehensive evaluation for irAEs in cancer immunotherapy. This abstract is funded by: none
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